The present invention relates to a color cathode-ray tube (hereinafter simply referred to as a color CRT) for use in, for example, a display unit of work station or data processor, and more particularly to an electron gun of this type of color CRT. The color CRT of this invention may also be used for other display units than the above.
As diagrammatically exemplified in FIG. 1, the color CRT includes a panel portion P provided with an image screen formed of a phosphor layer 100, a neck portion N for accommodating an electron gun 200, and a funnel portion F for connecting the panel portion P and neck portion N. Mounted on the funnel portion F is a deflection unit DY operable to scan electron beams B, emitted from the electron gun 200, on the phosphor layer 100 coated on the inner surface of the panel portion P.
The electron gun 200 accommodated in the neck portion N has various electrode units including a cathode electrode unit, a control electrode unit, a focusing electrode unit of a non-magnetic material and an accelerating electrode unit of a non-magnetic material which are arranged successively in a direction of the axis of the electron gun. The cathode electrode unit includes a plurality of cathodes and electron beams produced from the cathodes are each modulated by a signal applied to the control electrode unit, and formed into a requisite crossectional shape and supplied with requisite energy through the focusing electrode unit and accelerating unit so as to be caused to impinge on the phosphor layer 100. Midway between the electron gun and the phosphor layer, each electron beam is deflected horizontally and vertically by a magnetic field H generated from the deflection unit DY mounted on the funnel portion F so that a two-dimentional image to be displayed can be formed on the image screen. Denoted by M is a convergence adjusting magnet assembly.
The electron gun 200 is supplied with power in operation and its temperature rises considerably. Specifically, heaters of the cathode electrode unit generate heat, beginning with the start of operation of the color CRT and the temperature of the electron gun is raised by the heat. With the electron gun's temperature raised, electrodes forming the electron gun undergo a deformation due to the heat and relative position change and as a result, trajectories for the electron beams change.
More particularly, electrode deformation and a change in the relative positional relation between adjacent electrodes take place, and an electric field (electron lens) formed by the electrodes is distorted to prevent the electron beam from travelling on a requisite trajectory. This brings about degradation in focus characteristic and convergence characteristic of the color CRT.
For these reasons, the material of the electrodes is required to have a low thermal expansion coefficient in an environment for operation of the color CRT.
Further, in order for the electrodes not to affect the electron beam trajectory which is properly determined by means of the deflection unit (deflection yoke) and static convergence adjusting magnet, the electrode material is required to be non-magnetic in the operational environment of the color CRT.
In the past, the electrodes are in general made of stainless steel having a thermal expansion coefficient .alpha. of about 18.times.10.sup.-6 /.degree.C., and the aforementioned degradation in focus characteristic and convergence characteristic is not always suppressed sufficiently.
To cope with this problem, JP-A-60-95836 (laid-open on May 29, 1985), for example, discloses non-magnetic and low thermal expansion coefficient materials suitable for formation of the electron gun, the materials being alloys such as nichrome alloy (an alloy containing about 80 wt % of Ni and about 20 wt % of Cr or an alloy containing about 65 wt % of Ni, about 30 wt % of Cr and 1 or less wt % of Fe) and Inconel (an alloy containing about 72 wt % of Ni, about 18 wt % of Cr and 10 or less wt % of Fe).
Each of the above materials has a low thermal expansion coefficient, but disadvantageously, it is expensive by containing a great amount of Ni and is poor in machinability due to 20 wt % of Cr or more contained therein, leading to an increase in cost of production of the electron gun and color CRT.
JP-A-61-290635 (laid-open on Dec. 20, 1986) shows an electron gun for a color CRT in which a preceding stage electrode is made of a material having a thermal expansion coefficient smaller than that of a material of a succeeding stage electrode.